This study is designed to continue our observation that cells pretreated with methotrexate will have an augmented rate of 5-fluorouracil (5-FU) metabolism. This sequence results in synergistic killing of tumor cells. Current work is designed to evaluate the enzymes which are responsible for 5-FU metabolism in various tumor cell lines including human tumor cell cultures and the co-substrate, Phosphoribosyl-pyrophosphate (PRPP). This information would lead to a profile of biochemical determinates which might prove to be predictive of cytotoxicity for this drug sequence. Other inhibitors of purine synthesis, such as MMPR and Azaserine will also be evaluated.